1. Field of Invention
This invention generally relates to a fluid seal and more particularly relates to a brush seal for sealing a leakage path between relatively movable parts.
2. Description of Related Art
It is well known to use a brush seal to establish a fluid seal between relatively movable components. Typically, the seal comprises a pack of resilient bristles that are fixed to one of the components and are in sliding relationship with the other component. For instance, an annular brush seal positioned in a leakage path between a rotatable shaft and static structure surrounding the shaft. The seal bristles in such an application are normally bonded to a mounting ring in turn carried in the static structure. The bristles are generally radially inwardly directed so that their free ends engage the shaft in sliding relationship. Together, the bristle pack and the shaft surface cooperate to define a leakage barrier between a region of high fluid pressure and a region of low fluid pressure.
Under the influence of a pressure drop the bristles tend to deform. To counter this effect, a backing member is used to provide additional support for the bristles on the downstream, low pressure side of the bristles and extends from the mounting member alongside the downstream face of the bristle pack to terminate short of the free ends of the bristles, thereby providing the necessary degree of lateral support for the bristles.
A drawback in the use of such backing member is that as a result of the pressure drop that exists across them the bristles are urged into frictional engagement with the face of the backing member. The restoring forces provided by the inherent resilience of the bristles and other forces are thus inhibited. The loss of this restoring force is highly undesirable in view of the adverse effect that it can have upon sealing efficiency. For instance, in the above example if the shaft makes a temporary excursion from its normal position, the bristles will be deflected radially outwards and may be frictionally constrained from returning to their normal position. This will in turn result in a gap opening between the bristles and the shaft surface and consequently increased seal leakage. In similar circumstances some bristles adopt a more radial disposition and thus by being frictionally constrained, increase the contact pressure between the bristles and the rotor with consequent increase of bristle wear, increased leakage and reduced seal life.
EP0617216 Al describes a brush seal arrangement in which the problem of bristle restoration force loss is addressed. In that arrangement, the backing ring is spaced apart from the majority of the bristles so that any frictional engagement between them is minimized. However, if the pressure drop across the bristles is high, there is a danger that the bristles will distort laterally and frictionally engage the backing ring, thereby resulting in a loss of the bristle restoring force.